Drying process



Patented May 5, 1942 John V. Shinn, Flushin Miehener, Claymont, eral Chemical Company, NewYorlr, N.

g, N. Y., and Aubrey W.

ss n ors to Gencorporation of New York No Drawing. Application June 12, 1940,

. Serial No. 340,056

5Claims. (Cl. 159-49) This invention relates to methods for drying liquors to form substantially dry solid products recoverable from such liquors by heating of the same to eliminate vaporizable constituents. More particularly, the invention is directed to manufacture of substantially anhydrous disodium phosphate. Y

Anhydrous disodium phosphate is usually made by concentrating and dehydrating, in some way or other, a disodium phosphate liquor containing disodium phosphate in solution and in suspension. In modern practice, drum driers are used economically to eliminate water from many kinds ofslurries and liquors. One conventional type of drier comprises a rotatable horizontally mounted metallic drum equipped with suitable steam pipe 'and valve connections to provide forv internal steam heating. Adjacent the lowermost periphery of the drum is a trough placed lengthwise of the drum and into-which is run whatever liquor is to be dried. Relation of drum and trough are such that on rotation a portion of the drum surface becomes immersed in the liquor in the trough, and on emergence from the bath certain quantities of liquor adhere to the outer drum surface. The apparatus includes a longitudinal stripping knife arranged to contact a portion of the drying surface at apoint, just prior to resubmergence of such portion in the liquor bath. In this way, dried material is shaved oif the drier by action of the knife.

It has not been possibletosecure consistent commercially satisfactory results by drying runof-plant disodium phosphate liquors by customary operation of drum driers. dried product adheres tenaciously to the metal In many instances surface of the drum making necessary the application of excessive pressure to the knife in order to scrape ofi' dried product. liven heavy lmife pressure is ineffective, and in certain situations substantial quantities of material still adhere to the drum after passing the knife, indications being that some portions of the disodium phosphate become in a sense semi-fused to the metallic drum surface. Heavy knife pressure requires use of a large amount of power to turn the drier, and also results in rapid wearing away of the knife edge with necessity for frequent shutdowns for resharpening, realignment, or replacement.

These disadvantages give rise to high power costs and low production per unit drier. Further, the substantially anhydrous product obtained is in an unsatisfactory powder-like form.

In the course of work upon the problem presented, we have found that in commercial practice compositions of run-of-plant disodium phosphate liquors vary. While small and in most instances insufiicient to adversely afiect specific'ations of the final product, such variations are great enough to upset the particular conditions needed for satisfactory 'drum drying. We find that operating difliculties such as those enumerated may be overcome by feeding to the heating surface of the drying equipment a disodium phosphate liquor the composition of which lies within relatively specific limits. More particularly, we have discovered that the sought-for objects of the invention may be attained by feeding to the drying surface a disodium phosphate liquor con-' taining, by weight of the liquor, not more than 0.02% monosodium phosphate and not more than 0.05% trisodium phosphate, calculated on a 50 B. at 212 F. basis. By operating in accordance with the procedure of the invention as herein described, a large portion of the dried product of its own accord chips oil the drying surface in relatively flake form even beforethe material reaches the stripping knife, and remaining dried material may be scraped oil the drum by very low knife pressure.

The principles of the invention are applicable to drying of disodium phosphate liquors regardless of the particular mode of production. For example, the disodium phosphate liquor to be dehydrated may be made by melting the dodecahydrate in its own water of crystallization, and then concentrating the melt by heating. In large-scale operation. a suitable liquor to be dehydrated may also be the disodium phosphate liquor resulting from filtration, and subsequent concentration if necessary, 01' the liquor formed in the usual process for making hydrated disodium phosphate.

In practice we proceed by treating or adjusting the liquor to be dehydrated to form a mass of the composition indicated. -While treatment may be eil'ected at any time during make-up, the more practicable procedure is to first bring the liquor.

ordinarily by boiling weak liquor, to a concentration of about 50 Be. at 212 F., determine the monosodium or trisodium phosphate content by known tests, and then calculating the amount of treating material needed to bring the liquor within the indicated monosodium-trisodium phosphate range. Adjustment may be accomplished by adding to the liquor sodium hydroxide or sodium carbonate on the one hand, or by addition of phosphoric acid on the other. Where the monosodium phosphate content exceeds 0.02% by weight on a 50 B. at 212 F. basis. requisite quantities of sodium hydroxide or sodium carbonate may be added to reduce the monosodium phosphate content to not more than 0.02% on the same basis. In this situation it is preferred to employ sodium carbonate since even though an excess of sodium carbonate were used, practically no trisodium phosphate would .be formed, and hence there is no danger of forming a liquor containing more than 0.05 trisodium phosphate. In cases where the composition of the liquor is such that the trisodium phosphate content exceeds 0.05% on a 50 B. at 212 F. basis, we treat the liquor with a sufficient amount of preferably relatively pure phosphoric acid to reduce the trisodium phosphate content to not more than 0.05% on the same basis. Should the liquor to be dried have concentration of say 49 B. at 212 F. (instead of 50 B. at 212 F. as above indicated), determination of monosodium or trisodium phosphate content may be made as before. Conversion of such analysis to 50 B. basis may be had by multiplying such analysis by 50/49, and according to the value thus obtained the amount of treating material to be used'may be calculated.

Especially when dehydrating disodium phosphate liquor by means of drying equipment such as a drum drier or similar apparatus where the drying surface passes thru and picks up an adhering layer of the liquor to be dried, we find that to attain the best overall operating advantages another control factor becomes of importance. We prefer to carry out make-up of the disodium phosphate prior to drying so, that the liquor has a concentration of not less than 49 B. at temperatures in the range of 200 to 220 F. Formation of such a liquor may be effected by suitable control of the previously mentioned concentrating operation. We have found that by using a liquor ofconcentration not less than 49 B. at zoo-220 F., it is ssible to cause adherence to the drum surface emerging from the trough of a layer of disodium phosphateliquor of a thickness such that, in conjunction with the previously described composition control, there is produced dried material of such physical characteristics that a good portion chips (I the drum surface in flake form before such material reaches the scraping edge of the knife. In order to avoid formation, on the drum surface leaving the bath, of a liquor layer which because of its apparent thickness and inherent nature interferes to a more or less extent with 7 ready removal of the dried material from the drum, it is preferred to operate so that the concentration of the liquor in the trough and as fed to the drying surface is not in excess of 51 B. at temperature of 200 220 F. Temperature of the liquor in the feed trough and as actually fed onto the drying surface may vary considerably and is for the most part a matter of operating convenience, i. e., such temperature should be high enough to maintain the liquor in readily fiowable condition.

Prior to development of the invention it was found that when using a conventional drum drier and a run-of-plant liquor containing 0.07% mono sodium phosphate, from commercial viewpoint the results obtained with respect to power consumption, dried product removal, physical form of the product and knife readjustments and replacements were entirely unsatisfactory, and use of a drum drier for dehydration was impracticable. Further, experience shows dried material removal from drying surface became difficult and unsatisfactory where the trisodium phosphate content exceeded the indicated 0.05% by Weight. On the other hand, in one example of operation of the invention where the concentration of the liquor fed to the drum was about 50 B. at 212 F. and the monosodium phosphate content was 0.02%, much of the product chipped off the drum of its own accord in flake-like form before reaching the knife, and the knife cleaned the drum completely, the final product containing about 0.5% moisture.

The invention may be practiced in-connection with use of equipment other than a drum drier. For example, a heated metal belt drier, or,a rotating disk drier, or any other type of drier ordinarily including a knife for removal of dried material from the heating surface may be employed insteadof a drum drier.

We claim:

1. In the process for making substantially anhydrous disodium phosphate by procedure involving drum drying of run-of-plant disodium phosphate liquor containing a material of the group consisting of monosodium phosphate and trisodium phosphate, the improvement comprisin forming and feeding onto the drying surface of the drum a disodium phosphate liquor of concentration not less than 49 B. at temperature of 200-220 F. and containing by weight not more than 0.021% monosodium phosphate and not more than 0.05% trisodium phosphate, on a 50 B. at 212 F. basis.

2. In the process for making substantially anhydrous disodium phosphate by procedure involving drum drying of run-ofvplant disodium phosphate liquor containing a material of the group consisting of monosodium phosphate and trisodium phosphate, the improvement comprising forming and feeding onto the drying surface of the drum a disodium phosphate liquor of concentration of 49-51 B. at temperature of 200- 220 F. and containing by weight not more than 0.02% monosodium phosphate and not more than 0.05% trisodium phosphate, on a 50 B. at 212 F. basis.

3. In the process for makingsubstantially anhydrous disodium phosphate by procedure involving drum drying of run-of-plant disodium phos-.'

phate liquor containing a material of the group consisting of monosodium phosphate and trisodium phosphate, the improvement comprising forming and feeding onto the drying surface of the drum a disodium phosphate liquor containing by weight not more than 0.02% monosodium phosphate and. not more than 0.05% trisodium phosphate, on a 50 B. at 212 F. basis.

4. The process for making substantially anhydrous disodium phosphate from run-of-plant disodium phosphate liquor containing a material of the group consisting of monosodium phosphate and trisodium phosphate, which process comprises forming a disodium phosphate liquor of concentration of not less than 49 B. at temperature of 200-220" F. and containing by weight not more than 0.02% monosodium phosphate and not more than 0.05% trisodium phosphate, on a memo? phate and in'isudium phesphate, which gun-mess cumprises foaming a dismimm phosphate 11mm!" cuntaimng "w "weight not mom mm WWW/5; zrmmm sodium phosphaute and mfi name than @.@5% m sndium phosphate an a 50 at 212? mm, feeding such liquor, m him farm M a milmimiy thin layer, onto a heated mrmce, heating"; MM

1mm mmcimi", w mmmmmy efiehymmm the swam, mumm mmum mhstamamlly @mhydmm' 

